1. Field of the Invention
The present invention relates to an exhaust gas-purifying system.
2. Description of the Related Art
In some cases, a diesel oxidation catalyst, a diesel particulate filter, a selective catalytic reduction catalyst, and an ammonia slip catalyst are arranged in this order from the upstream side toward the downstream side in an exhaust system for a diesel engine.
In such an exhaust gas-purifying system, the exhaust gas discharged from the diesel engine passes through the diesel oxidation catalyst. The diesel oxidation catalyst promotes oxidation of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas. Thus, the concentrations of carbon monoxide and hydrocarbons in the exhaust gas are lowered, and the particulate matter (PM) made of high-boiling hydrocarbons and sulfides is partially removed therefrom.
Subsequently, the exhaust gas passes through the diesel particulate filter. The diesel particulate filter filtrates the exhaust gas with the porous walls thereof to remove the remainder of the particulate matter from the exhaust gas.
Into the exhaust gas having passed through the diesel particulate filter, ammonia (NH3) or a precursor of ammonia such as aqueous urea (CO(NH2)2) is injected. As described in JP-A 2008-296224, the selective catalytic reduction catalyst promotes the reaction of nitrogen oxides (NOx) with ammonia added to the exhaust gas or originated from the precursor of ammonia. By this reaction, the nitrogen oxides are reduced into nitrogen. As a result, the concentration of nitrogen oxides is lowered.
Then, the exhaust gas passes through the ammonia slip catalyst. The ammonia slip catalyst is an oxidation catalyst. The ammonia slip catalyst promotes oxidation of unreacted ammonia discharged from the selective catalytic reduction catalyst. Although oxidation of ammonia produces nitrogen oxides, an amount of nitrogen oxides produced in the ammonia slip catalyst is small because the amount of unreacted ammonia discharged from the selective catalytic reduction catalyst is small. That is, the ammonia slip catalyst lowers the concentration of ammonia in the exhaust gas without greatly increasing the concentration of nitrogen oxides in the exhaust gas.